Method of preparing phosphine



NOV. 5, 1963 GORDON 3,109,794

METHOD OF PREPARING PHOSPHINE Filed July 27, 1960 ios monus l6 1 iiliil- -l7 X J} CATHODE Q V United States Patent 3,109,794 METHOD OF PREPARING PHOSPHINE Irving Gordon, Niagara Falls, N.Y., assignor to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York Filed Italy 27, 1950, Ser. No. 45,667 14 Claims. (Cl. 204-401) This invention relates to the preparation of phosphine by the electrolysis of phosphorus.

Heretofore, phosphine has been prepared by the reaction of metallic phosphides or phosphonium halides with water, and by the hydrolysis of elemental phosphorus. These methods have been unsatisfactory because of the high production costs and/ or because the phosphine product is in an impure form.

United States Patent No. 1,375,819, issued April 26, 1921, to Henry Blumenberg, In, discloses a method for preparing arsine by the electrolysis of a salt or oxide of arsenic in the presence of sulfuric acid and potassium sulfate or other compounds capable of liberating nascent hydrogen upon electrolysis. However, phosphine is not produced under the conditions set forth by Blumenberg when an oxide or salt of phosphorus is employed. i

W. R. Grove, in the Journal of the Chemical'Sooiety, vol. 16 (1863), pp. 263-272, discloses the use of an electric current to boil moist molten phosphorus and produces phosphine thereby. Such a technique requires a high voltage, and converts only a small amount of phosphorus to phosphine.

It is an object of this invention to provide a method of H producing phosphine by electrolytic means.

Another object of the invention is to provide a more economical method of producing phosphine.

Still another object of the invention is to provide a method of producing phosphine in a form substantially free from phosphorus hydrides and other phosphorus impurities.

Still another object of the invention is to provide an electrolytic method of producing phosphine without employing a diaphragm between the anode and cathode areas. a

These and other objects of the invention will be apparent f'ro-m the following detailed description of the invention.

I have now discovered a process for preparing phosphine by electrolytic means wherein an electric current is passed between a carbonaceous anode and a cathode in contact with an electrolyte, at least a portion of the cathode being in contact with molten phosphorus. It is not necessary to employ a diaphragm or other means for separating the cathode and anode gases. Oxygen, which is produced at the anode during electrolysis, reacts with the carbonaceous anode to yield a gas predominating in carbon dioxide and containing only minute proportions of carbon monoxide and oxygen. The gaseous cathode prodnot, which predominates in phosphine and contains minor proportions of hydrogen, is combined with the anode gas in the electrolytic cell, and discharged therefrom.

The combined catholyte gas and anolyte gas may be employed as a reactant wherever phosphine may be employed as a reactant, as long as the carbon dioxide component of the gaseous mixture is inert in the reaction. However, if it is desired to remove the carbon dioxide, the mixed catholyte-anolyte gas may be passed through a caustic solution to separate carbon dioxide from the gas mixture. Since the catholyte-anolyte gas mixture is substantially free from oxygen there is no danger of oxidizing or burning the phosphorus in the cell, and there is no danger of oxidizing or burning the phosphine gas within or without the cell.

3,16%,794 Patented Nov. 5, 1963 The accompanying drawing is a schematic illustration of a typical electrolytic cell suitable for carrying out the novel process.

Referring to the drawing there is shown a cell vessel 10 provided with a gas-tight cover 11 provided with ports 12, .13, 14, 15 and 16.

A liquid cathode 17 such as mercury may be placed in the bottom of cell vessel 14} up to the level indicated by interface 18. Molten phosphorus 1!? is placed in contact with liquid cathode 17 up to the level indicated by interface 20. An aqueous electrolyte 21 fills the cell vessel up to the level indicated by interface 22.

Carbon'aceous anode 23 is inserted into the cell through port 13, the bottom end of the anode being positioned in the electrolyte 21 but above interface 20. Anode conductor 24- connects anode 23 with the positive pole of an electrical energy source 25. Cathode conductor 26 extends-from liquid cathode 17 through port 12 to the negative pole of electrical energy source 25. Suitable insulation material 32, such as glass, covers all but the lower end of cathode conductor 26 within the cell vessel.

When an electric current is impressed upon the system a phosphine-containing gas is generated at the cathode 1'7, and a carbon dioxide-containing gas is generated at the anode 23. The gas is combined in gas space 27 and discharged through gas discharge line 28 which extends through poi-t .14. The gas mixture discharged through gas discharge line 31 may be conveyed to storage, or passed through a conventional caustic scrubber (not shown) to remove substantially all of the carbon dioxide firom the gas mixture.

A fresh supply of molten phosphorus and/or liquid cathode and/or electrolyte may be introduced into cell vessel '10 by means of funnel 29 which passes from the cell vessel exterior through port 15 into the electrolyte. A motor driven impeller 30 or other suitable agitation means may be positioned in the bottom portion of the cell vessel to etfect agitation of the molten phosphorus and/ or the liquid cathode.

A temperature measuring means 31 such as a glass thermometer may be inserted through port 16 and the electrolyte 21 if desired.

It will be recognized by those skilled in the art that the design of the electrolytic cell shown in the drawing may be modified Without departing from the spirit of the invention. For example the cathode, which is shown in the drawing as a liquid cathode, may be replaced with a solid cathode. In the latter case the liquid components in the cell vessel will be comprised of a lower molten phosphorus layer and an upper aqueous electrolyte layer. When a solid cathode is employed, the molten phosphorous may be agitated sufficiently to maintain maximum contact between the solid cathode and the molten phosphorus. Cell vessel 10 may be constructed of any impervious material such as glass, ceramics, rubber-lined steel and the like.

Any material having a hydrogen overvoltage as normally measured in the absence of phosphorus exceeding the hydrogen overvoltage of smooth platinum may be employed as the cathode. Typical cathodic materials include lead, lead-mercury amalgam, tin, mercury, cadmium, copper, bismuth, aluminum, zinc, brass, silver, nickel, tellurium, Monel, gold, and alloys thereof. For example, the alloy known as Woods metal, which is an alloy containing fifty percent bismuth, twenty-five percent lead, twelve and one-half percent tin, and twelve and onehalf percent cadmium, may be employed. This alloy may be used in either liquid or solid form. Black phosphorus may also be employed as a cathode material.

It is desirable to employ a cathode in a form having a high unit of area per unit of weight. As indicated prea U viously, liquid cathodes may be employed. When a solid cathode is employed the cathode may have the form of a helical coil, wire gauze or screen, perforated sheets and the like.

Suitable anode materials include graphite, impregnated graphite, and other types of carbonaceous electrodes. The anode is preferably used in the form of a rod, but any suitable form may be used.

' Suitable electrolytes include aqueous solutions of phosphoric vacid, sulfuric acid, hydrochloric acid, sodium chloride, lithium chloride, sodium sulfate, potassium chloride, potassium sulfate, monosodium phosphate, disodium phosphate, and mixtures thereof. An aqueous phosphoric acid solution containing between about ten and about eighty, and preferably between about fifteen and about fifty percent phosphoric acid by weight, is preferably employed as the electrolyte, but other concentrations may be employed if desired. Concentration of the aqueous solutions of the aforesaid acids, when employed as an electrolyte, should be equivalent to the aforesaid phosphoric acid concentrations range. Aqueous solutions of the aforesaid salts having a concentration between about ten percent by weight and the concentration sufiicient to produce a saturated solution under the temperature conditions obtained, may be used.

Molten white phosphorus, sometimes referred to as yellow phosphorus, is preferably employed as the source of phosphorus for the production of phosphine, but other allotropic forms of phosphorus may be employed if desired. The temperature of the phosphorus should be sufficient to maintain it in a molten state, without effecting boiling thereof. For this reason, the temperature of the molten phosphorus and electrolyte is maintained within the range between about forty-four degrees and about two hundred and eighty degrees Centigrade, and preferably between about fifty and about one hundred and twenty degrees centigrade. Temperature control of the phosphorus and the electrolyte may be readily obtained by means of constant temperature hath (not shown in the drawing) surrounding cell vessel 10, butany suitable temperature control means may be employed. For example, on start-up of the electrolytic process, the molten phosphorus and electrolyte may be heated to a temperature within the aforesaid temperature range by means of an external source of heat, and maintained at this temperature by means of a constant temperature bath and/or the heat generated in the cell.

The rate of phosphine production and the purity of the phosphine product varies with the current and current density. When a low current density is employed, a gaseous mixture of phosphine and hydrogen is produced at the cathode, the resulting gas mixture containing a high concentration of phosphine. However, under these conditions, the production rate of the gas mixture is relatively low, being generally below the level which is considered economically feasible. When high current densities are employed the production rate of the phosphinecontaining gas is increased, but the concentration of phosphine is reduced. Increasing the cathodic current density will increase the production rate, but also reduces the phosphine concentration in the catholyte gas. It will be recognized by those skilled in the art that the optimum current and optimum current density will vary with the size and design of the cell, but in each case it is important to employ those conditions that yield a catholyte gas having a high concentration of phosphine consistence with commercially feasible production rates. For example, a cathodic current density within the range between about five and about one thousand amperes per square foot yield optimum results in most instances, but higher or lower current densities may be employed if desired. It is important to employ, in each case, current which will produce a voltage drop across the system. of less than about twenty volts and preferably less than about ten volts. -When the voltage drop is in excess of about i twenty volts, a significant proportion of the electrical energy imparted to the cell is wasted in heating the ingredients in the cell, instead of being utilized to effect electrolytic decomposition thereof.

The following examples are presented to define the invention more clearly without any intention of being limited thereby. All parts and percentages are by weight unless otherwise specified.

Example 1 An electrolytic cell was constructed from a five hundred I milliliter glass beaker and a gas-tight rubber stopper having a graphite anode, a copper wire cathode lead, a thermometer, and a glass tubing gas outlet. Fifty milliliters of mercury and thirty grams of molten white phosphorus were added to the glass beaker. About four hundred milliliters of aqueous eighteen percent phosphoric acid solution was added to the glass beaker to serve as an electrolyte.

Copper wire, which was connected through mercury to a tungsten lead immersed in the liquid cathode, was connected tothe source of direct current.

The anode which was a graphite rod six inches long and three-eighths inch diameter extended through the rubber stopper into the electrolyte but above the level of molten phosphorus. A copper lead connected the anode to the source of direct current. Agitation of the electrolyte, molten phosphorus, and mercury was effected by means of a plastic coated magnetic stirrer which was rotated at the rate of about two hundred revolutions per minute.

A current of three amperes and a voltage of 10.2 volts were impressed upon the system during electrolysis. The temperature of the electrolyte and molten phosphorus was maintained at about eight-five degrees ceutigrade by placing the cell vessel in a constant temperature bath.

The gaseous mixture of catholyte gas (predominating in phosphine and containing a minor portion of hydro gen) and vanolyte gas (predominating in carbon dioxide and containing a minor portion of carbon monoxide) was produced at the rate of about twenty-six milliters per minute. Analysis of the gaseous product showed a phosphine content of about 40.6 percent, a carbon dioxide content of 33.5 percent, and a hydrogen content of 25.9 percent by volume.

Example 2 The procedure of Example 1, employing the cell of Example 1, was repeated with the exception that the eighteen percent aqueous phosphoric acid electrolyte was replaced with forty percent aqueous phosphoric acid solution. The gaseous product was produced at the rate of about 16.5 milliliters per minute and contained about 69.7 percent phosphine by volume.

It will be recognized by those skilled in the art that various modifications within the invention are possible, some of which have been mentioned above. Therefore, I do not wish to be limited except as defined by the appended claims.

I claim:

1. A process for preparing phosphine which comprises contacting a carbonaceous anode and a cathode with an aqueous electrolyte, said cathode being in contact with molten phosphorus, passing an electric current between said anode and said cathode, whereby a gas predominating in phosphine is produced at the cathode and whereby a gas predominating in carbon dioxide is produced at the anode, and recovering the resulting gaseous mixture containing phosphine and carbon dioxide.

2. The process of claim 1 wherein said cathode is in agitated contact with said molten phosphorus.

3. The process of claim 1 wherein said cathode is mercury.

4. The process of claim 1 wherein said cathode is a liquid alloy of bismuth, lead, tin and cadmium.

5. The process of claim 1 wherein said electrolyte is capable of forming hydrogen ions under the electrolysis conditions employed and is non-reactive with molten phosphorus.

6. The process of claim 1 wherein said electrolyte is phosphoric acid.

7. The process of claim 1 wherein said electrolyte is an aqueous phosphoric acid solution containing between about ten and about eighty percent phosphoric acid by weight.

3. The process of claim 1 wherein the temperature of the electrolyte and molten phosphorus is maintained during electrolysis within the range between about forty-four and about two hundred and eighty degrees centigrade.

9. The process of claim 1 wherein the temperature of the electrolyte and molten phosphorus is maintained during electrolysis within the range between about fifty and about one hundred and twenty degrees centigrade.

10. The process of claim 1 wherein the current density of said cathode during electrolysis is maintained in the range between about five and about one thousand amperes per square foot. 1

11. The process of claim wherein the voltage during electrolysis is maintained below about twenty volts.

12. The process of claim 1 wherein said gaseous mixture containing phosphine and carbon dioxide is passed through an aqueous caustic solution, whereby the carbon dioxide component of said gaseous mixture is removed from said gaseous mixture, and recovering the resulting gaseous product predominating in phosphine.

13. A process for preparing phosphine which comprises passing an electric current between a graphite anode and a mercury cathode in contact with an aqueous phosphoric acid solution electrolyte, a portion of said electrolyte in contact with said mercury cathode being admixed with molten phosphorus, said molten phosphorus being in agitated contact with said cathode, maintaining a current density on said mercury cathode of at least about five amperes per square foot, whereby a phosphine containing gas is produced at the cathode, and whereby a carbon dioxide containing gas is produced at the anode, and recovering a gaseous mixture of said phosphinecontaining gas and said carbon dioxide-containing gas.

14. The process of claim 13 wherein said gaseous mixture of phosphine-containing gas and carbon dioxidecontaining gas is passed through an aqueous caustic solution to remove carbon dioxide from said gaseous mixture, and recovering a gaseous product predominating in phosphine.

References Cited in the file of this patent UNITED STATES PATENTS 1,375,819 Blumenberg Apr. 26, 1921 1,970,973 Palmaer Aug. 21, 1934 2,726,930 Edwards et al. Dec. 13, 1955 2,867,568 Cunningham Jan. 6, 1959 2,913,383 Topfer Nov. 17, 1959 FOREIGN PATENTS 1,130,548 France Oct. 1, 1956 OTHER REFERENCES Treatise on Powder Metallurgy, by Goetzel, volume HI, 1952, page 63 (#907).

Pauling, College Chemistry, 2nd edition, 1955, pages 330-5.

Ephraim, Inorganic Chemistry, 5th edition, 1948, pages 617-22.

Journal of The Chemistry Society, volume 16, 1863, pages 263-72. 

1. A PROCESS FOR PREPARING PHOSPHINE WHICH COMPRISES CONTACTING A CARBONACEOUS ANODE AND A CATHODE WITH AN AQUEOUS ELECTROLYTE,SAID CATHODE BEING IN CONTACT WITH MOLTEN PHOSPHORUS, PASSING AN ELECTRIC CURRENT BETWEEN SAID ANODE AND SAID CATHODE, WHEREBY A GAS PREDOMINATING IN PHOSPHINE IS PRODUCED AT THE CATHODE AND WHEREBY A GAS PREDOMINATING IN CARBON DIOXIDE IN PRODUCED AT THE ANODE, AND RECOVERING THE RESULTING GASEOUS MIXTURE CONTAINING PHOSPHINE AND CARBON DIOXIDE. 